1. Field of the Invention
The invention pertains to MOS (metal oxide semiconductor) integrated circuits and, more particularly, to integrated circuits in which the threshold voltage can be adjusted with precision.
2. Description of the Prior Art
MOS type integrated circuits use one or two types of transistors to achieve all the functions of the circuit. In the most recent technologies, these are n-type and p-type transistors. With these two types of transistors, it is very easily possible to fulfil all the logic functions, namely the functions using only logic gates and programmable logic arrays (PLAs).
The problems become more difficult to solve when the functions that have to be performed are outside the scope of standard logic. Now, most of the present-day circuits call for at least one variably analog function. This is the case, for example, with the power-on reset function. This is also the case with voltage level detectors, memory reading amplifiers, operational amplifiers, analog-digital or digital-analog converters, etc. All these circuits require analog reference voltages, and the problem lies in the precision of these references.
With standard integrated circuits, this precision is often very difficult to obtain owing to the variation in the parameters of the transistors depending on the different production batches from which they come. For, the working of a transistor in saturation mode can be described by an equation that expresses the drain current I.sub.D, as a function notably of the voltage between the gate and the source V.sub.GS, and of the threshold voltage V.sub.T of the transistor. All these parameters vary according to production batch, and it is impossible to achieve precise control of the value of the drain current as a function of the gate/source control voltage.
The problem is even more difficult when a circuit requiring high precision uses two types of transistors, namely transistors of the n-type and of the p-type. For, the electrical parameters of these two types of transistors may vary in opposite directions. This leads to major fluctuations in the working of circuits of this type. The parameter that has the greatest influence on the characteristics of the transistor is its threshold voltage. Now, this threshold voltage varies with the variation in the voltage of the source of the transistor. As a consequence, even if the threshold voltage V.sub.TO can be adjusted with precision when the source is connected to the ground, it is very difficult to obtain stable operating values in operating mode.
An object of the invention is a MOS cell in which the threshold voltage of the transistors is made adjustable.